FIG. 1 depicts a schematic diagram of a portion of a typical wireless telecommunications system, which provides wireless telecommunications service to a number of wireless terminals (e.g., wireless terminals 101-1 through 101-3) that are situated within a geographic region. The heart of a typical wireless telecommunications system is Wireless Switching Center ("WSC") 120, which may be also known as a Mobile Switching Center ("MSC") or Mobile Telephone Switching Office ("MTSO"). Typically, Wireless Switching Center 120 is connected to a plurality of base stations (e.g., base stations 103-1 through 103-5) that are dispersed throughout the geographic area serviced by the system and to the local- and long-distance telephone offices (e.g., local-office 130, local-office 138 and toll-office 140). Wireless Switching Center 120 is responsible for, among other things, establishing and maintaining calls between wireless terminals and between a wireless terminal and a wireline terminal, which wireline terminal is connected to Wireless Switching Center 120 via the local and/or long-distance networks.
The geographic area serviced by a wireless telecommunications system is divided into spatially distinct areas called "cells." As depicted in FIG. 1, each cell is schematically represented by a hexagon; in practice, however, each cell has an irregular shape that depends on the topography of the terrain surrounding the cell. Typically, each cell contains a base station, which comprises the radios and antennas that the base station uses to communicate with the wireless terminals in that cell and also comprises the transmission equipment that the base station uses to communicate with Wireless Switching Center 120.
For example, when wireless terminal 101-1 desires to communicate with wireless terminal 101-2, wireless terminal 101-1 transmits the desired information to base station 103-1, which relays the information to Wireless Switching Center 120. Upon receipt of the information, and with the knowledge that it is intended for wireless terminal 101-2, Wireless Switching Center 120 then returns the information back to base station 103-1, which relays the information, via radio, to wireless terminal 101-2.
Typically, the signal transmitted by a wireless terminal to a base station is radiated omni-directionally from the wireless terminal. Although some of the signal that is transmitted radiates in the direction of the base station and reaches the base station in a direct, line-of-sight path, if one exists, most of the transmitted signal radiates in a direction other than towards the base station and is never received by the base station. Often, however, signals that radiate initially in a direction other than towards the base station strike an object, such as a building, and are reflected towards the base station. Thus, a signal can radiate from the wireless terminal and be received by the base station via multiple signal paths.
FIG. 2 depicts an illustration of wireless terminal 101-1 as it transmits to base station 103-1. Signal 107-1 is received by base station 103-1 directly via a line-of-sight path. Signal 107-2, signal 107-3 and signal 107-4 arrive at base station 103-1 after radiating initially in a direction other than towards base station 103-1 and only after reflecting off of an object, such as buildings 105-2 through 105-4, respectively. Signals 108-1 through 108-4 radiate from wireless terminal 101-1 but never reach base station 103-1.
Because each of the four signals arrives at base station 103-1 after having traveled a different path, each of the four signals arrives at different times and interfere to form a composite of the four constituent signals. This is known as the multipath phenomenon. And furthermore, depending on the length of the path traveled and whether the signal is reflected off of an object before reaching base station 103-1, the signal quality (as measured by, for example, the average power of an amplitude-modulated signal, the signal-to-noise ratio, absolute power in dBm, etc.) of each signal is different when received. This is partially due to the fact that when a signal is reflected off of an object, the extent to which the signal is attenuated is a function of, among other things, the angle at which the signal is incident to the object and the geometric and dielectric properties of the object.
In a code-division multiple access ("CDMA") wireless telecommunications system each radio receiver endeavors to identify and isolate the highest-quality constituent signals in the composite multipath signal and to demodulate and combine them to form an estimate of the transmitted signal. As is well-known in the prior art, this process is conducted with, among other things, a rake receiver. A rake receiver analyzes a composite signal, in well-known fashion, and attempts to identify the strongest constituent signals in the composite signal. The rake receiver then isolates and demodulates each of the strongest constituent signals, and then combines them, in well-known fashion, to produce a better estimate of the transmitted signal than could be obtained from any single constituent signal. To accomplish this, a rake receiver comprises a plurality, but finite number, of "fingers," each of which isolates and demodulates one constituent signal.
Because each constituent signal travels a different path from the transmitter to the receiver, it is highly unlikely that the distance traveled by all of the constituent signals will be exactly the same. Any discrepancy is manifested as a relative time-delay, or phase-shift, in the constituent signals. Any phase-shift in a constituent signal that does not exactly equal an integral number of wavelengths of the carrier signal is manifested by a partial phase rotation in the constituent signal with respect to the other constituent signals.
When the modulation scheme of the transmitted signal does not function by modulating the phase of the carrier, the partial phase rotation of the constituent signals at the receiver is irrelevant and does not affect the demodulation process. In contrast, when the modulation scheme of the transmitted signal functions, at least in part, by modulating the phase of the carrier signal (e.g., quadrature phase-shift keying, quadrature-amplitude modulation, etc.), the partial phase rotation of the respective signals must be considered in the demodulation process. Typically, the partial phase rotation of the respective signals is accounted for by re-aligning their phase, which makes quasi-coherent combination of the constituent signals possible.
In the prior art, one technique has been developed, called "pilot-aided CDMA," to facilitate the task of re-aligning the phase of the respective constituent signals. In a pilot-aided CDMA system a pilot signal is transmitted in the same channel as the information-bearing signal. Typically, the pilot signal has the same frequency as the information-bearing signal but has an invariant phase. The pilot signal is subject to the same environmental factors as the information-bearing signal as it traverses each path from the transmitter to the receiver, and, therefore, the multipath phenomenon acts on the pilot-signal in the same fashion as the information-bearing signal to ensure that a constituent pilot signal arrives at the receiver with each constituent information-bearing signal. Because each constituent pilot signal traverses the same path as its associated constituent information-bearing signal, each constituent pilot signal experiences the same phase rotation at the receiver as the associated constituent information-bearing signal. And because the rake receiver knows that the phase of the pilot signal, as transmitted, is invariant, the rake receiver can estimate the relative phase rotation of the respective constituent pilot signals, and can, therefore, estimate the phase rotation in each constituent information-bearing signal.
In a pilot-aided CDMA system in the prior art, the task of phase aligning the respective constituent information-bearing signals is performed by multiplying each constituent information-bearing signal by a factor called the "conjugate pilot estimate." The conjugate pilot estimate is created by the rake receiver based on the relative phase rotation of the constituent pilot signals. The products of each constituent information-bearing signal--conjugate pilot estimate pair are, therefore, phase-aligned, which enables quasi-coherent combination, in well-known fashion, and the estimation of the originally transmitted information-bearing signal.
Pilot-aided CDMA is well-known in the prior art and the reference CDMA: Principals of Spread Spectrum Communication, Andrew J. Viterbi, Addison-Wesley Publishing Company, 1995, pp. 87-96, is a typical reference on the topic.
FIG. 3 depicts a block diagram of the salient components in an pilot-aided CDMA receiver in the prior art. CDMA receiver 300 comprises: antenna 301, radio front-end 302 and rake receiver 305. Rake receiver 305 typically comprises a bank of N fingers, 307-1 through 307-N, each of which outputs a constituent information-bearing signal, I.sub.i (n), and an associated conjugate pilot estimate, P.sub.i (n), for i=1 to N, wherein n indicates the temporal sequence of the received signals. Each constituent information-bearing signal, I.sub.i (n), and its associated conjugate pilot estimate, P.sub.i (n), are multiplied by a conjugate pilot multiplier, and quasi-coherently combined by combiner 312, in well-known fashion, to provide an estimate, I(n), of the originally transmitted information-bearing signal.